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Measuring Distance

When I look at specifications on throw distances of sprinklers, I'm
a little confused if they are talking about (thinking in terms of a
right triangle) the distance in terms of the hypotenuse or the cosine
(normal horizontal ground measurement) with respect to the angle set by the sprinkler rotor's valve angle. Is their some general rule to follow with respect to knowing the sprinkler's horizontal area of coverage; rather than having to break open my old physics books and factor in parameters such as spray angle, nozzle pressure, ect.....
I want to use a Hunter I-20 rotor throwing at 16 feet. Also, do
these spray distances apply to spray sprinkler systems (horizontal
radius).

The radius of throw of a rotor or spray head is measured in feet and describes the distance the water will be thrown forward, away from the sprinkler head itself. Anotherwords, if the I-20 rotor specs state that the radius of coverage is 17' to 47', that means the furthest distance the water will be thrown will be 47' and the shortest distance will be 17' feet. The different distances or radius are achieved by installing smaller or larger nozzles into the rotor head (complete nozzle sets come with each rotor) and by turning down or up an adjustment screw on the rotor head.

The Arc is the amount of a circle that the rotor will rotate to provide coverage ( how much of a circle is covered by the rotor spray). The arc is very simple to increase or decrease. The left or right side of a rotor, depending on which brand you choose, will be the fixed side that you line up with the boundary (sidewalk). You can then use the adjustment screw on the rotor to open or close the arc thus increasing or decreasing the degree of a circle covered.

I do not suggest you use an I-20 rotor for throwing 16 feet. You should use a rotor that is designed for that small of throw, Hunter PGM rotor or Rain Bird 3500 series or 5004 series rotor. The I-20 would have to be turned down too much which would mess up the uniformity of spray.